"JC" virus has been isolated by Padgett et al. (1971) from a human brain with progressive multifocal leukoencephalopathy (PML), a progressively fatal demyelinating disease occurring as an opportunistic infection. The proposed studies are aimed chiefly at collecting data on the oncogenic poteential of this nw human papova virus. Gross, light microscopic and ultrastructural studies will be conducted on Syrian hamsters which have been inoculated with "JC" virus by various routes. Our previous studies indicate that this virus is capable of producing malignant tumors, such as medulloblastomasof the brain, in an unusually high percentage of animals. These in vivo studies will be accompanied by in vitro studies of "JC" virus-induced cerebral or extracerebral tumor tissue. The morphology of virus particles, originally obtained from different human brains with PML and subsequently cultured in human fetal glial cells, will be studied by electron microscopy.